1. Field of the Invention
The present invention relates to a method of controlling the rotation speed of a motor to obtain a desired value and, more particularly, to an improved method capable of maintaining the rotational speed at a desired value without being affected by any ripple component contained in a speed detection signal outputted from a shaft encoder.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional apparatus for digital speed control of a DC motor disclosed in the "DC Motor Control and Sensor" made public in the 1983 Joint Meeting of Four Electric Societies. In this diagram, there are shown a digital speed reference signal 1, a speed controller 2 having a proportional-plus-integral function, a current controller 3 having also a proportional-plus-integral function, a gate pulse generator 4, a main circuit 5 of a thyristor converter, a DC motor 6, a current sensor 7 for detecting a main circuit current and outputting it in the form of a digital value, a shaft encoder 8 connected to one end of a shaft of the DC motor, and a speed sensor 9 for digitizing the detected speed value obtained from the shaft encoder. In this example, each of the speed controller 2 and the current controller 3 is supposed to have a computing function with a microprocessor employed therein.
Referring to the operation performed in the above exemplary apparatus, its control system comprises a speed feedback control circuit having a current minor loop, in which a deviation between the speed reference signal 1 and the speed feedback value obtained from the speed sensor 9 is inputted to the speed controller 2 having a proportional-plus-integral function, where the speed value is computed. The output of the speed controller 2 serves as a current reference signal, and a deviation between this reference signal and the current feedback signal detected by the current sensor 7 is inputted to the current controller 3. The output of the current controller 3 serves as a phase reference signal for the gate pulse generator 4 and, in response to occurrence of any speed variation, a steady state is achieved when a coincidence is attained between the speed detection value and the speed reference signal.
Describing now the method for such speed detection, the output of the shaft encoder 8 contains some ripple (detection error) resulting from the mechanical precision, condition of installation and so forth of the shaft encoder. Although the ripple component is normally .+-.0.1% or so, it is amplified, when the encoder output is fed to the speed controller, at a proportional gain and is thereby increased to a current ripple on the order of 1 to 10%, which then brings about limitation relative to setting of the gain in the speed control system. In order to minimize the speed detection error, several methods are adopted today including one that averages the detection value by taking a longer speed sampling time, and one that obtains an average value by the use of known detection data acquired empirically in the past. However, a disadvantage is unavoidable due to a prolonged idle time in the control system so that a fast control response cannot be obtained.
The general speed detection method in the prior art is described in the cited reference.